In an earlier blog post about 3D printing, “Bearings Technology Risks in Precision Linear Motion System Development,” we examined bearing technology options; now we conclude by looking at available technologies that will ensure the proper positioning of production equipment components: position feedback systems.

Position feedback systems essential

Without position feedback, a 3D printing or Additive Manufacturing (AM) device might print a defective part. For example, if the system is instructed to move to index up 0.1 mm and to print as the system scans to the left 10 mm, then the part produced should be 0.1 mm thick and 10 mm long. If the system doesn’t move correctly, the parts produced will be out of tolerance. The longer this error persists unnoticed, the more cost and time are lost due to printing replacements for the defective parts. Including a position feedback system on every axis of motion prevents printing in the wrong location by ensuring proper location of the print head at all times.

There are two design options for position feedback systems,

Rotary

Linear.

Rotary designs typically incorporate an optical or magnetic encoder mounted to the rear shaft of the motor driving the axis. While suitable for most general AM applications, there is risk in that feedback is not on board the carriage, but instead on board the motor, which gives a close, but not completely accurate location of the print head. Drive train wind up or backlash, motor shaft wind up, and other coupling flexibilities result in the print head being slightly removed from the expected position, even when the system is shut off. This risk can be reduced by minimizing all components between the carriage or print head and the rotary encoder or resolver. For example, with a screw-driven solution, the encoder should be mounted directly to the opposite end of the screw from the motor. This will eliminate motor shaft wind up and coupling backlash, leaving only the backlash occurring between the carriage and the screw and the screw wind up.

For constant precise location of the print head, the solution is to use the linear feedback design. Linear encoders or transducers mount a head to the carriage of the moving component and a scale or readable device to the stationary component. As the carriage moves, the read head will provide feedback of exact location based on the scale.

The risks with the linear encoder are similar to other technologies; it must maintain a certain allowable read distance from the scale. When this distance is violated, the position feedback will fault. To minimize this risk, mount the encoder to a system that has good flatness and straightness and also ensure that the bearings are traveling in a flat and straight manner.

There are risks associated with various types of linear encoder designs. Optical linear encoders (as well as the optical rotary encoders) are vulnerable to contamination. Since the read head visually inspects lines on the scale, contamination can impair the reading and fault the position sensors. This risk can be minimized and eliminated by ensuring that the debris in the environment doesn’t contaminate the scale. An alternative to this optical design is a magnetic design. The magnetic design is more resistant to contamination, unless, of course, the contamination is magnetized debris.

Table: Position Feedback System Technologies Summary

Position feedback system technologies summary

Open Loop
Inadvisable because while this design adds no cost or space requirements, it provides no position feedback and cannot determine if the system is building a layer correctly. If this system design is selected, it is imperative to ensure the affected motors have adequate torque to overcome stiction and/or system wear over time.

Rotary Encoder
Less expensive, with fewer moving cables and smaller space requirements than linear encoders. However, they are less accurate than linear encoders because they do not directly mount to the point of interest. It is imperative to eliminate the backlash and minimize the windup between the point of interest and the rotary encoder.

Linear Encoder
If the added expense can be overcome, linear encoders are the most accurate position feedback systems. The design includes a head mounted to the carriage of the moving component and a scale or readable device mounted to the stationary component. The risks with this design are similar to other technologies. A linear encoder needs to maintain a certain allowable read distance from the scale or readable device. If the distance is violated, the position feedback will fault and create location issues.

Risks associated with 3D motion systems

There are many risks associated with the technologies that build up 3D motion systems for additive manufacturing. The risk of printing or building bad 3D parts can be mitigated by incorporating position feedback into the system. A feedback mechanism will allow programming controls to ensure dispense or sintering does not happen if a position window is not satisfied.